SR-MPLS: fixes and tests

- the FIB path takes a vector of type fib_mpls_label_t not u32 so the untype safe vec_add did not work
- write som eSR-MPLS tests
- allow an MPLS tunnel to resolve through a SR BSID

Change-Id: I2a18b9a9bf43584100ac269c4ebc286c9e3b3ea5
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/mpls/mpls.api b/src/vnet/mpls/mpls.api
index 6047d25..7ab0f37 100644
--- a/src/vnet/mpls/mpls.api
+++ b/src/vnet/mpls/mpls.api
@@ -54,6 +54,7 @@
   u8 mt_next_hop_preference;
   u8 mt_next_hop[16];
   u8 mt_next_hop_n_out_labels;
+  u32 mt_next_hop_via_label;
   u32 mt_next_hop_sw_if_index;
   u32 mt_next_hop_table_id;
   vl_api_fib_mpls_label_t mt_next_hop_out_label_stack[mt_next_hop_n_out_labels];